When they did the inside the CMP episode on Shooting USA. They showed a Garand captured during the Vietnam War and another from Afghanistan. So the Army can turn over weapons from a variety of sources!!

The non USGI sporting arms on auction are often sourced by US military hunting clubs that have closed or just replaced equipment. The rifles that were provided to hunters on military posts were MWR owned equipment and therefore US Govt. Property. It is then normal for them to be disposed off through the arsenal system like Anniston which then passes it off to CMP. Its sad but there are fewer and fewer of these fine military shooting and hunting clubs that used to be a great perk of military service. My local experience was the Fall deer season every year at Camp Bullis, Texas (an extension of Fort Sam Houston north of San Antonio). There were some excellent deer hunts up there.